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Akka Actor Communication

In Akka, actors communicate to each other by sending and receiving messages.


Akka Actor Send Messages

Akka provides two predefined methods tell() and ask() for message exchange. An actor can send messages to another Actor through these following methods.

1) Akka Actor tell() Method

It is used to send a message asynchronously. It does not wait and block thread for a message. It works on "fire-forget" approach. You can also use ! (bang) exclamation mark to send message. This is the preferred way of sending messages. It gives the best concurrency and scalability characteristics.

If this method is invoked from within an Actor, the sending actor reference will be implicitly passed along with the message.

If this method is invoked from an instance that is not an Actor, the sender will be deadLetters actor reference by default.


Actor tell() Method Example

Output:

Message received: Hello from - RootActor
sender:Actor[akka://ActorSystem/deadLetters]  // ActorRef refers to deadLetters
Message received: Hello from - RootActor
sender:Actor[akka://ActorSystem/deadLetters] // ActorRef refers to deadLetters

Akka Actor tell() Method Example2

Output:

Message received: Hello from - RootActor
Sender: Actor[akka://ActorSystem/deadLetters]	// Called from outside Actor
Message received: Hello from - ChildActor
Sender: Actor[akka://ActorSystem/user/RootActor#1451914889]	// Called from within Actor

2) Akka Actor ask Method

In akka, ask is a pattern and involves Actors as well as Futures. Ask is used to sends a message asynchronously and it returns a Future which represents a possible reply. If the actor does not reply and complete the future, it will expire after the timeout period. After timeout period, it throws an TimeoutException. You can use either ? (question mark) or ask() to send message.

You should always prefer tell method for performance and ask method, if you want response.


Akka Actor ask Method Example

Output:

Message recieved: Hello
Exception in thread "main" java.util.concurrent.TimeoutException: Futures timed out after [2 seconds]

Akka Actor ask() Method Example2

Output:

Message received: Hello from outside actor instance
Replaying
Message received: Hello, I got your message.

Akka Actor Ask Method Example3

Output:

Message received: Hello from outside actor instance
Exception in thread "main" java.util.concurrent.TimeoutException: Futures timed out after [2 seconds]





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